The answer can be found on Apple's Forums. Also PHP is turned off by default, so be sure to edit the httpd.conf file to turn it back on.
From the post:
"I got PHP working with Leopard by modifying the httpd.conf file that you can get to by going to Go -> Go To Folder, /etc then going into the apache2 folder and copying httpd.conf to the Desktop (it won't let you edit in place). Find the line that says LoadModule php5_module etc...... and remove the # from the start. Save the file and drag it back into the apache2 folder, you'll have to authenticate to get it in there. Then restart apache by switching personal web sharing off and on in the sharing pref pane.
I had the same problem with my personal web sharing folder for my username not working on both the machines I installed it on (as an upgrade). The machines web sharing is working tho, just not the one for each user account. To fix it, create a file called shortusername.conf (where shortusername is your shortusername, eg, joebloggs and in it put this...
Options Indexes MultiViews
Allow from all
Save the file and put it into the apache2/users folder - restart personal web sharing and boom!, its working now."
I did all this in the terminal, sudo of course.
I hope this helps.